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Subsystems and network elements in GSM

Ciphering

Both the BTS and the MS must be able to cipher and decipher information in order to protcct the transmitted speech and data in the air intcrface.

Speech Processing

Speech processing refers to all the functions the BTS performs in order to guarantee an error-free connection between the MS and the BTS. This includes tasks like speech coding (digital to analoguc in the downlink dircction and vice versa), channel coding (for error protection), interleaving (to enable a secure transmission), and burst formatting (adding information to the coded speech / data in order to achieve a well-organised and safe transmission).

The base station can contain several TRXs (Transceivers), each supporting one pair of freąuencies for transmitting and receiving information. The BTS also has one or morę antennas, which are capable of transmitting and receiving information to/from one or morę TRXs. The antennas are either omnidirectional or sectorised. It also has control functions for Operation and Maintenance (O&M), synchronisation and extemal alarms, etc.

TC(SM) - Transcoder (SubMultiplexer)

In the air interface (between MS and BTS). the media carrying the traffic is a radio freąuency. To enable an efFicient transmission of digital speech information over the air interface, the digital speech signal is compressed. We must however also be able to communicate with and through the fixed network, where the speech compression format is different. Somewhere between the BTS and the fixed network, we therefore have to convert from one speech compression format to another, and this is where the Transcoder comes in.

For transmission over the air interface, the speech signal is compressed by the mobile station to 13 Kbits/s (Fuli Ratę and Enhanced Fuli Ratę) or 5.6 Kbits/s (Half Ratę). The compression algorithm for Fuli Ratę is known as "Regular Pulse Excitation with Long Term Prediction" (RPE-LTP). For Enhanced Fuli Ratę, speech coding is based on the algorithm "Algebraic Codę Excited Linear Prediction" (ACELP). "Vector Sum Excited Linear Prediction" (VSELP) is used in the case of Half Ratę. However, the standard bit ratę for speech in the PSTN is 64 Kbits/s. The modulation techniąue is called "Pulse Codę Modulation" (PCM).

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